1. Field of the Invention
The present invention relates to a closed cell alkenyl aromatic polymer foam containing a low solubility chlorine-free hydrofluorocarbon blowing agent, a process for preparing such a foam and a method for using such a foam.
2. Description of Related Art
Chlorine-free hydrofluorocarbon (HFC) blowing agents are desirable for use in preparing polymer foam because they offer both low thermal conductivity and low environmental hazards in regards to ozone depletion. However, chlorine-free HFCs that are available in commercially viable quantities tend to have little solubility in alkenyl aromatic polymers such as polystyrene. Therefore, use of such chlorine-free HFCs in polystyrene foam is typically limited to less than eight weight-percent HFC based on total polymer foam weight in order to obtain a foam having good physical properties (for example, that is closed celled, has a density of 48 kilograms per cubic meter (kg/m3) or less and preferably is free of blow holes and preferably has a good skin quality).
PCT publication WO 98/03581 and U.S. Pat. No. 6,255,364 disclose preparing alkenyl aromatic foam with a blowing agent containing 70-95 wt percent (%) 1,1,2,2 tetrafluoroethane (HFC-134). Both references disclose an ability to achieve a closed cell foam having a density less than 48 kg/m3 using HFC-134 as a principle blowing agent. HFC-134 is a particularly attractive blowing agent because it has a relatively high solubility in alkenyl aromatic polymers. As WO 98/03581 notes, higher blowing agent solubility in a resin is advantageous by providing conditions for foam formation which are conducive to closed cell production over a wide range of densities while minimizing the possibility of blown/ruptured cells (see page 12, lines 22-27). Unfortunately, HFC-134 is not readily available on a commercial market, leaving less soluble chlorine-free HFCs as commercially viable blowing agent options.
Blowing agents that are less soluble than 134, however, are not especially conducive as blowing agents. For example, 1,1,1,2-tetrafluoroethane (HFC-134a) is a chlorine-free HFC that is commercially available but that is less soluble in polystyrene than HFC-134. Examples exist demonstrating use of HFC-134a as a blowing agent for styrenic foams but only at concentrations up to about 7.5 wt % based on polymer weight. (see, for example, European Patent 1214372B1). Use of HFC-134a at concentrations above 7 wt % reportedly produces poor quality foam due to blowholes, low expansion, open cells and a high nucleation rate. (as reported in a presentation by Bertrand Kieken entitled “Processability of HFC-134a in PS Foaming: Effect of Resin Structure” at the FoamTech Meeting dated 6 Dec. 2001). Foams are reportedly “hardly attainable” when using a blowing agent consisting of HFC-134a at concentrations above 7.5 wt %. (see, R. Gendron, et al., “Foam Extrusion of PS Blown with a Mixture of HFC-134a and Isopropanol”, JOURNAL OF CELLULAR PLASTICS 23(1) 2004, 1-23). Such problems are common with blowing agents having a low solubility in the polymer they are foaming.
U.S. Pat. No. 6,274,640 ('640) discloses a method for increasing solubility of HFC-134a in polystyrene for producing polymer foam by including an alcohol as a co-blowing agent. '640 discloses blowing agent compositions that contain from 4-8 wt % HFC-134a based on polymer weight.
U.S. Pat. No. 6,063,823 ('823) discloses that the solubility of an HFC into polystyrene increases upon blending into polystyrene an additional polymer containing oxygen, nitrogen or fluorine. However, '823 is clear to point out that blending more than 30 wt % of additional polymer into polystyrene provides no further improvement and in fact causes adverse effects to the physical properties of the resulting foam.
PCT publication WO 98/32787 offers a method to improve HFC solubility in an olefinic polymer (in which they include polystyrene) by adding a polar polymer having a low molecular weight and ability to dissolve the blowing agent when heat plasticized. The publication focuses primarily on acrylates as suitable polar polymers and specifically limits the amount of polar polymer to 50 wt % or less of the total polymer blend.
An opportunity exists to advance the art of insulating foam technology by identifying a process for preparing a closed cell, low density alkenyl aromatic polymer foam using a blowing agent comprising more than 30 wt % of a readily available chlorine-free HFC such as 1,1,1,2-tetrafluoroethane (HFC-134a) for a total chlorine-free HFC concentration of 8 wt % or more while maintaining quality foam physical properties.